1. Field of the Invention
The present invention relates to an initial correction factor calculating device used for a tire pressure drop detecting device, for example, for finding an initial correction factor for eliminating the effect on the rotational velocities of the tires of a difference in effective rolling radius that depends on an initial difference between tires.
2. Description of Related Art
In recent years, as an example of a safety device of a four-wheel vehicle such as an automobile or a truck, devices for detecting the drop in air pressure of a tire (DWS) have been developed, and some of them have-been put to practical use.
An example of a method of detecting the drop in air pressure of a tire is a method utilizing a difference among the respective rotational velocities F.sub.1, F.sub.2, F.sub.3, and F.sub.4 of four tires W.sub.1, W.sub.2, W.sub.3, and W.sub.4 mounted on a vehicle. This method utilizes the fact that the rotational velocity F.sub.i (i=1, 2, 3, 4) changes depending on the conditions of the air pressure of the tire W.sub.i. That is, when the air pressure of any one of the tires W.sub.i drops, the effective rolling radius of the tire W.sub.i decreases. As a result, the rotational velocity F.sub.i of the tire W.sub.i increases. Therefore, the drop in the air pressure of the tire W.sub.i can be judged on the basis of a difference in the rotational velocity F.sub.i.
The effective rolling radius is a value obtained by dividing by 2.pi. the travel distance of the tire W.sub.i when freely rolling one rotation in a loaded state.
A judgment expression used in detecting the drop in the air pressure of the tire W.sub.i on the basis of the difference in the angular velocity F.sub.i is the following equation (1): ##EQU1## (see, for example, Japanese Patent Laid-Open No. 305011/1988, Japanese Patent Laid-Open No. 212609/1992)
If all the effective rolling radii of the tires W.sub.i are the same, the respective rotational velocities F.sub.i are the same (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4). Consequently, a judged value D is zero. Therefore, thresholds D.sub.TH1 and D.sub.TH2 (where D.sub.TH1, D.sub.TH2 &gt;0) are set. When the condition given by the following expression (2) is satisfied, it is judged that there is a tire W.sub.i whose air pressure drops. When the condition is not satisfied, it is judged that all the tires W.sub.i have normal internal pressure. EQU D&lt;-D.sub.TH1 or D&gt;D.sub.TH2 (2)
The effective rolling radius of the actual tire W.sub.i includes a variation within a production tolerance (hereinafter referred to as an "initial difference"). That is, even if all the four tires W.sub.i have normal internal pressure, the effective rolling radii of the four tires W.sub.i differ depending on the initial difference. Correspondingly, the rotational velocities F.sub.i of the tires W.sub.i vary. As a result, the judged value D may be a value other than zero. Therefore, it may be erroneously judged that the air pressure drops, although it does not drop. In order to detect the drop in the air pressure with high precision, therefore, it is necessary to eliminate the effect of the initial difference from the detected rotational velocity F.sub.i.
As a technique for eliminating the effect of the initial difference from the rotational velocity F.sub.i, it is considered that a technique disclosed in Japanese Patent Laid-open No.318584/1995 is applied. In the technique, the ratio of the rotational velocities of right and left following (rear) tires is calculated. Further, a time-differentiated value of the ratio of the rotational velocities is calculated. It is judged whether or not the time-differentiated value remains at not more than a predetermined limit value over a predetermined time period. If the time-differentiated value remains at not more than the limit value over the predetermined time period, it is judged that the vehicle is traveling linearly. An initial correction factor is calculated on the basis of the rotational velocities F.sub.i calculated at this time.
The initial correction factor is calculated only when it is judged that the vehicle is traveling linearly. Therefore, the calculation is not affected by the difference between the rotational velocities F.sub.i of inner and outer tires which occurs at the time of cornering. Therefore, an initial correction factor faithfully representing a variation in effective rolling radius which depends on an initial difference between tires W.sub.i, should be calculable.
In the above-mentioned prior art, however, a time-differentiated value of the ratio of the rotational velocities of right and left following tires is used as a basis for judging whether a vehicle is traveling linearly. Therefore, it may, in some cases, be erroneously judged that the vehicle is traveling linearly, although it is cornering.
Specifically, when the vehicle is traveling on a road having an almost constant radius of curvature (hereinafter referred to as a "constant-R road"), for example, an exit road leading to a tollgate from an exit of a main lane in an interchange of a highway, it is erroneously judged that the vehicle is traveling linearly. More specifically, when the vehicle is traveling on such a constant-R road, a time-differentiated value of the ratio of the angular (rotational) velocities is relatively small, and is hardly changed. In this case, it is judged that the time-differentiated value of the ratio of the angular velocities remains at not more than the limit value, and it is erroneously judged that the vehicle is traveling linearly.
In this case, therefore, an initial correction factor which is affected by a variation in the rotational velocity F.sub.i caused by cornering is calculated, whereby the rotational velocity F.sub.i is erroneously corrected. As a result, the drop in air pressure of a tire is erroneously judged.